Fluoropolymers, i.e., polymers having a fluorinated backbone, have long been used in a variety of applications because of several desirable properties, including thermal stability.
Fluoroelastomers in particular, perfluoroelastomers, exhibit significant tolerance to high temperatures and harsh chemical environments. Various cure systems are known to cure perfluoroelastomers. See, for example, John B. Marshall, “Kalrez®-Type Perfluoroelastomers—Synthesis, Properties and Applications”, in Modern Fluoropolymers, John Scheirs, editor, John Wiley & Sons Ltd., New York, (2000) p. 351-353. Nitrile containing perfluoroelastomers are typically cured by trimerizing the nitrile groups into triazine crosslinks using a variety of catalysts such as ammonia generating compounds like those described in U.S. Pat. No. 6,281,296, imidate catalysts such as those described in U.S. Pat. No. 6,657,013, amidine cure systems such as those described in U.S. Pat. No. 6,846,880, and fluoroonium cure systems such as those described in U.S. Pat. No. 7,294,677 and U.S. Pat. No. 6,890,995. The most notable cure systems for perfluoroelastomers are the peroxide cure systems, such as described in U.S. Pat. No. 4,983,680, U.S. Pat. No. 7,388,054, and U.S. Pat. No. 6,465,576, and the triazine forming cure systems.
The cure system selected can also influence the chemical resistance and thermal stability of the fluoroelastomer. For example, the most thermally stable perfluoroelastomers are cured with a triazine-forming cure system, but these perfluoroelastomers are known to have poor chemical resistance (e.g., volume swell) and are not recommended for use in hot aliphatic amines, ethylene oxide, propylene oxide, and hot water/steam applications. See, for example, John B. Marshall, “Kalrez®-Type Perfluoroelastomers—Synthesis, Properties and Applications”, in Modern Fluoropolymers, John Scheirs, editor, John Wiley & Sons Ltd., New York, (2000) p. 351-352. On the other hand, perfluoroelastomers cured with peroxides in the presence of co-agents, such as triallyl isocyanurate, are traditionally known for their chemical resistance, but lack thermal stability. See, for example, John B. Marshall, “Kalrez®-Type Perfluoroelastomers—Synthesis, Properties and Applications”, in Modern Fluoropolymers, John Scheirs, editor, John Wiley & Sons Ltd., New York, (2000) p. 351-352.
There has been a desire in the industry to develop a perfluoroelastomer composition that achieves both chemical resistance and thermal stability. The compositions, known in the art at the time of filing, which attempt to achieve both chemical resistance and thermal stability are O-rings sold under the trade designation “KALREZ SPECTRUM 6375”, which are said to “give outstanding performance in the widest possible range of chemicals and temperatures” and polymer gum sold under the trade designation “TECNOFLON PFR 95”, which offers “the broadest working temperature range and widest resistance to chemical media”. See “KALREZ SPECTRUM 6375” product bulletin H-82112-01 printed October 1999, and “TECNOFLON PFR 95” product bulletin revised February 2003, respectively.
Another method to improve the performance of the perfluoroelastomer composition is to combine the triazine-forming and peroxide cure systems to take advantage of the chemical resistance of the peroxide cure and the high temperature resistance of the triazine-formed cure. However, there is a trade-off in the performance (i.e., the mixed cure system does not perform as well as or as poor as either cure system individually). Thus, one needs to determine the optimal ratio of triazine to peroxide curatives to balance the performance characteristics and ultimately the mixed-cure perfluoroelastomer composition has a performance somewhere in between the two cure systems individually. Thus far, it has not been possible to satisfy all the desired properties for a particular sealing application, which requires both high temperature performance and broad chemical resistance.